PROJECT SUMMARY Survival rates after lung transplantation continue to lag substantially behind those observed after transplantation of other solid organs. A major barrier to success of lung transplantation is primary graft dysfunction, a form of acute lung injury that is mediated by ischemia reperfusion injury. Primary graft dysfunction not only results in an increase in early morbidity and mortality, but it also predisposes to chronic lung allograft dysfunction. We have developed new clinically relevant mouse models of lung transplantation and novel intravital imaging approaches to examine mechanistic links between primary graft dysfunction and rejection. In our previous funding period we have discovered that monocytes play a central role in mediating primary graft dysfunction by regulating neutrophil entry into the reperfused lung graft. Furthermore, we have described mechanisms how neutrophilic uptake of endogenous ligands that are released from injured lungs propagates primary graft dysfunction. In this application we propose to examine pathways that result in the activation of monocytes and neutrophils in lung grafts and to uncover mechanisms how these cells promote alloimmunity and graft rejection. In Aim 1 we will define mechanisms that activate neutrophils upon graft infiltration. In Aim 2 we will analyze pathways that mediate the activation and differentiation of monocytes in lung grafts. In Aim 3 we will examine interactions between monocytes and T cells that can trigger graft rejection.